The present invention relates to a system for quantifying the degree of crazing in plastic transparencies, such as those used for aircraft windscreens and passenger cabin windows.
Aircraft transparencies and commercial aircraft cabin windows tend to craze if subjected to mechanical or chemical stress. Crazing is the phenomena that results in tiny microcracks in a transparency surface. In general, the microcracks are perpendicular to the surface and are usually aligned in a structured pattern corresponding to the direction of applied stress. The microcracks are wide enough that the two sides of the crack are not in optical contact, and therefore behave like tiny mirrors and reflect light according to the law of reflection. The visual effect of crazing is that the transparency will suddenly illuminate if a light source (such as the sun) is in a correct position with respect to the eye position where the law of reflection is satisfied. If no light source is in the connect position with respect to the eye, the transparency may exhibit only a minor haze effect and may be otherwise substantially totally transparent. The crazing phenomena is therefore somewhat insidious in that it occurs only under certain conditions. No commercial system is available for quantitatively measuring the degree of crazing in transparencies with any acceptable degree of reliability, and it is highly desirable to have a system for quantifying crazing in aircraft transparencies for replacement when safe limits of crazing are exceeded.
The invention meets the need as just suggested by providing a system for quantitatively measuring crazing in plastic transparencies. The system of the invention includes a diffuse light source disposed on one side of the transparency and a calibrated photometric light detector on the other side arranged to capture and quantify the amount of light reflected from the microcracks comprising the crazed condition. The amount of reflected light is proportional to the degree (quantity and size of microcracks) of crazing in the transparency.
The invention can easily be configured as a portable system and facilitates a determination of the amount of crazing in an aircraft transparency without removing the transparency from the aircraft. The system is highly sensitive to the orientation of the microcracks.
The invention may find substantial use on aircraft to determine when crazing has reached a level requiring replacement of the transparency, for establishing limits on crazing for removal of critical passenger windows used by flight crews for inspecting the leading edge of wings for icing, and for setting basic FAA safety standards regarding crazing in commercial airline windows.